This invention relates to the fields of printed circuit boards and integrated circuit chip packaging and mounting.
A variety of techniques exist for packaging integrated circuit chips and mounting them on a printed circuit board. Perhaps the most widely used technique is that of encapsulating a chip in epoxy or enclosing it in a ceramic package. With this technique, the chip is first mounted at the center of a plurality of radially extending leads. Then, fine wires are welded onto wire bonding pads on the chip. The opposite end of each of these wires is welded to the inner end of one of the radial leads. This process for electrically connecting the chip to the leads with fine wires is called "wire bonding." The chip and the inner end of each radial lead are then encapsulated in epoxy or enclosed in ceramic, with the outermost end of each lead being left exposed. The exposed ends of the leads are bent downward so that they may be plugged into an integrated circuit chip socket or soldered directly onto the printed circuit board. In this way, the chip is electrically and mechanically coupled to the printed circuit board.
This method of mounting and packaging integrated circuits has several advantages. For example, integrated circuit chips are occasionally damaged when wires are welded to the wire bonding pads on the chip surface. Therefore, it is important to test the chips after they are wire bonded to the leads. With this method, each chip may be tested after the chip is encapsulated in the epoxy or enclosed in the ceramic package, but before it is connected to the circuit board. If the chip does not function properly, it can be discarded and replaced with a functional chip. Furthermore, if a fully assembled circuit board should fail to function properly, the defective chip can be easily removed from its socket or unsoldered from the board and replaced with a functioning chip.
The epoxy and ceramic packaging materials also serve to protect the chip from moisture and other deleterious external influences which could corrode the wire bonded connections. However, the disadvantage of this technique is that each individually packaged chip takes up a relatively large amount of space on the printed circuit board. Thus, the density of individually packaged chips that can be put on a board is limited by the external dimensions of the chip packaging.
Another method for mounting integrated circuits on printed circuit boards is called the "chip-on-board" method. In the chip-on-board method, an unpackaged integrated circuit chip is glued or soldered directly onto the surface of the printed circuit board. The chip is then wire bonded directly to the printed conductive filaments (called "traces") of the printed circuit board. To protect the chip, which would otherwise be exposed to ambient conditions, the chip and wire connections are then covered with a drop of epoxy.
The chip-on-board technique can be cheaper than the previously described individual encapsulation method since the chip-on-board method does not require individual protective capsules for each chip. Furthermore, because individual capsules are not used, each chip mounted in this way takes up much less space on the printed circuit board than an equivalent encapsulated chip. Thus, chips mounted with the chip-on-board method can be more densely packed onto a board.
The chip-on-board technique, however, has several disadvantages. As previously mentioned, chips are occasionally damaged during the wire bonding process. Thus, since it is generally impractical to test each individual chip immediately after the chip is wire bonded to the board, testing must be delayed until all of the individual integrated circuit chips are mounted on and wire bonded to the board. Then, the entire board must be tested as a single functioning unit. This procedure results in a comparatively high percentage of defective boards since the probability of obtaining a functioning board is no greater than the product of the probabilities that each of the chips on the board is functional.
In addition, boards with chips mounted in this way cannot be economically repaired since the defective chips would have to be disconnected and replaced manually. This is a very expensive procedure. Thus, the chip-on-board technique can be used economically only for devices where the percentage of functioning boards is high and where the entire assembled board is relatively inexpensive so that few boards need to be discarded and the boards which are defective can be discarded without undue expense.
In contrast to these previously known techniques, the present invention combines the pre-test advantages of the individual chip encapsulation method with much of the space saving advantages of the chip-on-board technique.